1. Field of the Invention
The present invention relates to a pin (i.e., cylindrical member) for absorbing neutrons, which is an element or component of a reactor control rod (or to a reactor control rod which has been assembled by using such an element). The pin according to the present invention is capable of retaining not only a good mechanical characteristic but also a good compatibility between plural components constituting the pin, even up to a high temperature.
2. Related Background Art
In a nuclear reactor such as fast breeder reactor, it is extremely important to control the operating state or condition of the nuclear reactor (e.g., to control the output of the reactor, to carry out an emergency shutdown, etc.). In order to effect such a control, the reactivity of the reactor(or the degree of the reaction occurring in the reactor) is changed, usually by putting into the reactor a control rod including a neutron absorber (or neutron absorbing material) as a constituent thereof, and taking the control rod out of the reactor in an appropriate manner.
Many of the above-mentioned control rods have a structure comprising an assembly of a plurality of "neutron absorbing pins" bundled together, and each of the neutron absorbing pins comprises a cladding (or covering tube) comprising stainless steel, and a neutron absorber (such as boron carbide (B.sub.4 C) pellet) inserted therein. In the neutron absorbing pin, the neutron absorber comprising a boron carbide pellet, etc., generally has a cylindrical shape having a high density, which has been sintered by hot-pressing, etc. Usually, a small gap portion is provided between the pellet and the stainless steel cladding.
In many cases, the gap portion has a structure such that it is filled with helium gas, or liquid sodium for the purpose of lengthening the life of the control rod.
The former type of neutron absorbing pins are referred to as "helium-bonded pin", and the latter type of neutron absorbing pins are referred to as "sodium-bonded pin".
It has been known that, during the use of a neutron absorbing pin within a nuclear reactor, when a boron carbide pellet constituting the pin is irradiated with neutrons, .alpha..sup.10 B(n, .alpha.).sup.7 Li reaction occurs, and helium (He) produced from the reaction is accumulated in particles of the boron carbide, whereby the pellet causes swelling (volume expansion).
The change in size or dimension of the boron carbide pellet in the axial direction of the neutron absorbing pin caused by the swelling is usually absorbed by a pellet-pressing spring which is disposed between an end of the pin and the pellet in the control rod in the axial direction thereof, whereby such a phenomenon little causes a problem substantially.
However, when the size or dimension of the boron carbide pellet is increased in the diametrical direction, thereof the gap between the boron carbide pellet and the stainless steel cladding is decreased, whereby the boron carbide pellet and the cladding would finally come into mechanical contact with each other. When the neutron absorbing pin is further used in such a state successively, the degree of the mechanical interaction between the pellet and the cladding is further increased, and the cladding begins to be deformed plastically. When such a plastic deformation exceeds the limit of deformation or strain, there occurs an accident such that the cladding is finally damaged or broken.
In addition, the above-mentioned boron carbide pellet has a tendency such that it is liable to be broken into fine fragments mainly due to a thermal stress occurring in the pellet, while the boron carbide pellet is used in the reactor. When the boron carbide is broken into fine fragments, there occurs a phenomenon such that the fragment of the pellet is moved or relocated in the cladding, or a small fragment enters a crack or cleavage in the pellet, or the gap between the pellet and the cladding. When such a phenomenon occurs, the gap initially provided between the pellet and the cladding is rapidly filled (closed), after the initiation of the irradiation thereof with neutrons. As a result, the mechanical interaction between the boron carbide pellet and the cladding will occur at a point of time which is earlier than that has preliminarily been expected, and the life of the cladding of the neutron absorbing pin (or control rod) is shortened.
In consideration of these circumstances, for the purpose of preventing the fragments of boron carbide from moving in the cladding, there has been proposed a structure wherein a thin-wall pipe (usually, referred to as a "shroud") comprising austenite stainless steel, ferrite steel-stainless steel, etc., is disposed in the above-mentioned stainless steel cladding so that the shroud covers the entire length of the neutron absorber pellet (see, Japanese Patent Publication (KOKOKU) No. Hei 6-31769 (i.e., 31769/1994); and B. T. Kelly et al., "International Conference on Fast Reactors and Related Fuel Cycles," Proceedings Volume III, p 1. 10-1, November 1991).
In the neutron absorbing pin equipped with such a shroud, the fragment of boron carbide is not moved in the cladding, and the neutron absorber is swollen while substantially retaining its original shape, whereby a longer life of the pin can be achieved until the mechanical interaction between the pellet and the cladding is initiated.
When a further long life is intended to be realized in such a neutron absorbing pin equipped with a shroud, it is theoretically possible to lengthen the life of the pin, e.g., by increasing the initial gap between the boron carbide pellet and the cladding. In this case, however, when the gap is increased, the heat transfer efficiency or rate is decreased between the pellet as a heat-generating member under neutron irradiation, and the cladding cooled by a coolant, whereby the temperature of the boron carbide pellet is elevated. Accordingly, the temperature of the shroud pipe becomes too high, and there occurs a phenomenon that the shroud comprising stainless steel becomes unusable.
In order to solve the above-mentioned problem, there has been adopted a measure such that the heat transfer efficiency is enhanced at the gap portion by putting liquid sodium in the gap portion between the cladding and the pellet constituting the neutron absorbing pin (i.e., to cause the pin to have a "sodium-bonded" structure), whereby the surface temperature of the pellet and the working temperature of the shroud are lowered. However, the sodium-bonded pin inevitably poses a problem such that it is difficult to handle the pin in view of the treatment (or disposal) of the sodium at the time at which the control rod is to be produced, the absorbing pin is to be discarded after the use thereof in the reactor, etc. In consideration of these circumstances, it has strongly been demanded to develop a neutron absorbing pin including as a constituent a heat-resistant shroud tube comprising a material other than stainless steel, e.g., in the case of above-mentioned "helium-bonded pin" having excellent characteristics in view of the production and handling thereof.
Further, for example, even in the case of the sodium-bonded pin, when the pin is used for a long period of time, the shroud comprising stainless steel is reacted with the boron carbide pellet, so as to cause carburization, etc., and to loose ductility thereof, whereby the performance or function as a shroud tube is inevitably lowered. Therefore, it has also been demanded strongly to develop a neutron absorbing pin including, as a constituent, a shroud material excellent in compatibility with boron carbide.